Broad Definition of FRC
Typically, the fibre reinforcement in an FRC consists of materials having a very high strength-to-weight ratio such as carbon, glass and certain polymeric materials such as polyaramid (Kevlar) or polyethylene (Dyneema) or organic materials such as Bamboo. These are produced as “tows” of long fibres or as short staples either in the form of spun yarns or randomly arranged, but in all cases are bound together by a matrix material made of polymers either in the form of thermosetting adhesives such as epoxy or polyester resins or as thermoplastic materials such as polyether ether ketone (PEEK) or Nylon and the like. The fibres generally carry the principal stresses to which the final material is to be subjected, and the matrix material carries the shear stresses which arise because of stress gradients within the structure. In some cases, the matrix may be a relatively strong material such as a ceramic, and the fibres are used in relatively small quantities to impede crack propagation in the final material, but more normally, the fibres constitute a large proportion of the final structure and the matrix simply acts to bind the fibres together.
Issues with FRC Products
FRC structures can be manufactured in various ways. The process used may depend on a number of factors including the volume, the quality of product required and the complexity of the final shape. To optimise strength/weight performance, the reinforcing fibres should be precisely positioned in accordance with the anticipated stress. For products which are not simple beams or plates and for high performance products, this often requires complex and expensive arrangements which may still not always achieve maximum performance. Practically, most economically viable manufacturing processes impose significant limitations on the reinforcing fibre arrangements. For example, in many processes, fibres are formed in layers in the final structure. Such layered or laminated structures are prone to delamination, wherein the laminates separate at the interface between two layers as a result of shear stresses.
A further issue with existing manufacturing techniques is that the process itself imposes stresses on the fibres and causes damage to them, resulting in significant loss of strength.
A further issue is that restrictions imposed by the assembly processes result in low fibre fill ratios. Since the fibre and matrix materials have comparable density, any excess volume occupied by the matrix material contributes to the weight but little to the strength.
A further issue is that the manufacturing processes usually require uniform distributions of fibre (for example, weaving produces homologous layers of material), but the stresses are unlikely to be so distributed. Consequently, the weights of tows and mats have to be designed to carry the maximum stresses arising, and this means that there will be excess material at all locations other than those subjected to the maximum stress.
One known method of manufacturing a composite material is by 3D weaving, whereby the threads are woven to form principle axes and planes of reinforcement and the resulting voids are later filled with—for example—foam. The resulting composite structure can have good all-round properties; in particular, the structural cell consisting of fibres laid in the principle directions for carrying direct stresses and fibres at ±45° for carrying shear stresses is versatile and allows the building of highly effective structural components. However, 3D weaving processes, while economically viable for higher performance applications, still impose restrictions on fibre alignment arising from the topology of the weaving machine and process. A further major limitation with the weaving process is the inability to vary the thickness of fibre along the fibre direction. Thus, 3D weaving of composite products is mostly used for essentially linear structures of constant cross section and carrying simple bending stresses.
An alternative process, braiding, inter-plaits three orthogonal sets of yarn (a length of interlocked fibres). Braiding forms intermediary structures, or tubes, used to produce the finished structure, the tubes being formed with both torsional and bending stiffness along their lengths, but again without the option to vary the size of the tube.
The as woven fibre structures tend to lack rigidity in themselves (until the matrix material has been injected and/or cured) and usually do not adequately define the overall shape of the product. So, additional material or components and complicated handling procedures are frequently required to define the overall shape.
Examples of Complex Products
In order to perform their function, some structures must carry stresses in two directions and torsion along their lengths, as well as having very particular shape requirements. For example, turbine blades require that the net shape of the aerofoil section varies and twists along the lengths of the blades. The principal stresses arise from bending both along the span and along the chord, together with torsion along the span due to the varying air pressure distribution as the sections change. Most of the stresses are carried in the skin, with the internal space largely void, but the structure needs to be stabilised with occasional webs connecting the skins both in the chord section (ribs) and along the span, and these webs need to be able to resist the buckling and shear stresses which arise between the outer skins. Thus, the overall requirement for arranging the fibres in an FRC turbine blade require them to be laid in at least two directions within the (curved) shape of the skins and at least two more directions forming intersecting webs internally. All of the sections and stresses vary along the length, calling for variable amounts of fibre throughout the structure.
Another example of high performance products is aircraft seating. Seats for aircrafts must satisfy particular standards, particularly in terms of impact resistance. For example, a dynamic crash resistance test subjects the seats to forces of up to 16 g. In order to meet these requirements, aircraft seats are typically produced using a large number of parts, including more recent use of composite materials to reduce the weight. Manufacturers have also tried to reduce the number of parts required; the more parts used, the more complex and costly the logistics and process of manufacturing such seats. Furthermore, the fastenings for these parts are often metal to be able to satisfy the safety requirements, increasing the weight of the seats. Moreover, attachment points such as these introduce stress concentrations and changes in stress direction that are difficult to transfer efficiently into the main load bearing members. Producing one piece aircraft seats using rotational moulding has been proposed, but the heating and cooling processes can be costly and there are limitations on the materials that can be used and precision with which shapes can be formed. It is also beneficial to reduce the volume of aircraft seats to be able to fit more seats onto an aircraft and/or increase the space allotted to each individual.
These are just two examples of a wide variety of applications that would benefit from the ability to produce high performance composite products in an efficient manner, with no or few design constraints imposed on the final product by the production process and with minimum stress applied to the fibres during the processing.
There is thus seen to be a need for new synthetic composite products and methods of manufacture that remove some of the constraints of existing techniques; permit the economic production of complex or high performance products; and/or enable the use in composite products of materials or combinations of materials not previously thought suitable.